1. Field of the Invention
The present invention relates to a rotating electrical machine such as a motor or a generator.
2. Description of Related Art
There is an ever-increasing need for the development of viable technologies that reduce CO2 emissions in order to address the issue of global warming. Among such technologies, high-efficiency rotating electrical machines, such as motors and generators, and electric vehicles, realized in conjunction with smaller rotating electrical machines capable of providing increased torque, attract our attention as potential means for CO2 emission reduction.
In Japan, motors, which are regarded as the “the industrial kernel”, account for approximately 70% of power used at factories. This means that by improving the motor efficiency by a few percent, a very significant energy saving, equivalent to several hundred thousand kilowatts of power generated at a power plant, could be achieved, which, in turn, would contribute to reducing CO2 emissions by several millions of tons per year. In addition, concerted efforts are being made toward further electrification of vehicles and promotion of consumer awareness of eco-friendly options such as HEVs (hybrid electric vehicles) and EVs (electric vehicles) as means for reducing CO2 emissions from transportation vehicles. For instance, an HEV, which requires only half the fuel used by a conventional gasoline-powered vehicle, also achieves greatly reduced CO2 emissions. Furthermore, vehicle electrification achieved by, for instance, motor-driven power steering instead of the conventional hydraulic power steering, improves fuel efficiency by 3 to 5% due to start-stop systems, and thus contributes to CO2 emission reduction.
The issues discussed above form a backdrop to the direction of resources aimed toward development of technologies that will improve the efficiency of rotating electrical machines (motors and generators) and more compact rotating electrical machines capable of generating greater torque (with higher efficiency) primarily geared to automotive use, so as to ultimately contribute toward creating a society with a lighter carbon footprint.
A more compact rotating electrical machine capable of generating larger torque and assuring high efficiency may be achieved by mounting a coil at a stator core with higher density. The joule heat generated as power is supplied to the stator core results in energy loss in the rotating electrical machine. This means that the efficiency of the rotating electrical machine can be improved by reducing the electrical resistance at the starter coil. In addition, the level of electromagnetic force is calculated as the BIL product (B: magnetic flux density, I: current, L: conductor length) and thus, a greater motor output can be provided by increasing the number of turns with which the stator coil is wound. In other words, a given output can be provided by a smaller motor. Accordingly, it is essential that a coil mounting structure be devised that will allow the conductor density (number of conductor lines X area) within a slot to be improved.
Such a coil mounting structure may be achieved through the use of a stator constituted with stator core segments or through the use of rectangular wire as the winding material. Compared to a stator with a single-piece core in the related art, a stator that adopts a concentrated winding method and includes a stator core constituted with core segments allows a greater number of coil turns and also allows the use of thicker wire, and therefore, a more compact rotating electrical machine, capable of providing greater output and assuring higher efficiency, can be achieved in conjunction with such a stator. In addition, by using a base wiring material constituted with rectangular wire instead of round wire, the cross-sectional area of the conductors that can be disposed within the slots can be increased, which, in turn, makes it possible to reduce ohmic loss at the rotating electrical machine.
When forming a coil with rectangular wire at a core segment wound with a concentrated winding method, a bobbin, constituted of an insulating material, is first mounted at the core segment and then the rectangular wire is wound at the bobbin. However, the rectangular wire cannot be wound at the bobbin in a trefoil formation, whereby non-angular wire, such as round wire, is stacked in a staggered pattern, and thus, the individual wire wraps tend to become misaligned (shifted laterally) during the winding process. Japanese Laid Open Patent Publication No. 2004-350450 discloses a structure intended to prevent such misalignment by forming stages at the bobbin.